1. Field of the Invention
The present invention generally relates to bridge DCxe2x80x94DC converters incorporating high voltage generating circuits of cathode ray tubes (CRTs). More particularly, the present invention relates to a bridge DCxe2x80x94DC converter in which the switching frequency to stabilize an output voltage during light loading is set at a frequency greater than the resonant frequency, which is defined by the inductance of a primary coil and the interwinding capacitance of a secondary coil of a converter transformer, so that the switching frequency during light loading may be significantly higher than that during heavy loading, whereby an exciting current component which flows to the primary coil is reduced to significantly enhance the energy conversion efficiency during light loading.
2. Description of the Related Art
In recent years, attempts have been made to use, as high voltage generating circuits for generating a high voltage applied to the anode of a cathode ray tube (CRT), asynchronous high voltage generating circuits which uses a frequency asynchronous with the horizontal scanning frequency as a switching frequency.
This is because asynchronous high voltage generating circuits which use a switching frequency much higher than the horizontal scanning frequency have several benefits compared to high voltage generating circuits which use a switching frequency synchronous with the horizontal scanning frequency. That is, the circuit components constituting the asynchronous high voltage generating circuit may be compact, and the cost of the overall circuit can be reduced. Furthermore, the higher the switching frequency, the lower the exciting current required. Thus, the energy conversion efficiency can be enhanced.
Such an asynchronous high voltage generating circuit is often implemented by, for example, a half bridge DCxe2x80x94DC converter. FIG. 6 illustrates the conceptual structure of a half bridge DCxe2x80x94DC converter 10. A dc power source 12 is connected to a switching unit 14 having a pair of switching devices, and the switching unit 14 is connected to a series circuit comprising a capacitor Cr, an inductor Lr, and a primary coil 22a of a transformer 22, which form a resonant circuit 20.
A secondary coil 22b of the transformer 22 is connected to a load 26 via a smoothing and rectifying circuit 24. The load 26 may be a CRT. When the load 26 is a CRT, the smoothing and rectifying circuit 24 may be implemented by a voltage multiplier rectifier circuit, where a high voltage of on the order of 20 to 30 kV is applied to the anode terminal of the CRT.
The high output voltage is supplied to an error detector 28, where it is compared to a reference voltage and the error voltage is supplied as a switching signal to a variable oscillator 30 to output an oscillation frequency corresponding to the error voltage. The oscillation frequency is supplied to the switching unit 14 via a driver 32. Therefore, the switching frequency which is made variable according to the load would achieve a stabilized output voltage.
In this structure, the resonance of the resonant circuit 20 is used to transfer electromagnetic energy to the secondary of the transformer 22 to provide a predetermined high output voltage HV. Herein, interwinding capacitance Cs of the secondary coil 22b which is present at primary coil 22a would be parallel to the primary coil 22a. The interwinding capacitance which is present in the primary is indicated by Cp in FIG. 6.
The relationship between the resonance characteristic when the interwinding capacitance Cs is present in the primary and the switching frequency is shown in FIG. 7. As in FIG. 6, in view of the interwinding capacitance Cs, the resonant circuit 20 would be a complex resonant circuit in which a series resonant portion comprising the capacitor Cr, the inductor Lr, and the inductance Lp of the primary coil 22a is combined with a parallel resonant portion comprising the inductor Lr, the inductance Lp, and the capacitor Cp.
The resonance characteristic is such that a first peak provided by the series resonant portion, that is, a resonance curve having a series resonant point Ps, is combined with a second peak provided by the parallel resonant portion, that is, a resonance curve having a parallel resonant point Pp. The high output voltage profile is higher when the load 26 is light-loading, while the high output voltage profile is lower when it is heavy-loading, thus proving a different resonance characteristic depending upon loading, i.e., heavy lording or light loading. That is, the resonance characteristic is represented by a curve La during light loading, while the resonance characteristic is represented by a curve Lb during heavy loading. The resonance curve varies between La and Lb depending upon load values, thereby providing a stabilized output voltage.
If a voltage for stabilization has been determined as depicted in FIG. 7, then, switching frequencies f2 and f4 corresponding to the predetermined voltage are obtained during light loading in a frequency region higher than the series resonant frequency fs and lower than the parallel resonant frequency fp, and a frequency region higher than the parallel resonant frequency, respectively. Switching frequencies f1 and f3 are obtained during heavy loading in the former and latter frequency regions, respectively.
The switching frequency of the half bridge DCxe2x80x94DC converter 10 is generally set higher than the resonant frequency fs corresponding to the series resonant point Ps. In this case, therefore, it is chosen to be within either frequency region Wa ranging from f1 to f2 or Wb ranging from f3 to f4. For example, the switching frequency is chosen to be within the frequency region Wa.
The electric current which flows to the resonant capacitor Cr and the resonant inductor Lr of the resonant circuit 20 shown in FIG. 6 is a combination of the current component which is transferred to the secondary and the current component which flows only to the primary, namely, the exciting current component. The exciting current component is a current component which does not contribute to electromagnetic energy transfer. The lower the switching frequency, the higher the amplitude of the exciting current component, and energy dissipation increases accordingly, as known in the art.
In the conventional DCxe2x80x94DC converter 10, as shown in FIG. 7, the switching frequency is operable in the frequency region Wa which is higher than the series resonant frequency fs. Here, there are only a few differences between the switching frequency f2 during light loading and the switching frequency f1 during heavy loading.
Specifically, for example, if the required high output voltage is 32 kV, this voltage corresponds to a predetermined voltage for stabilization, where when the turns of the secondary coil 22b are set at 500 T, the number of turns in the primary coil 22a is 30 T in the converter transformer 22. In this example, fs, f1, fp, and f2 are 50 kHz, 60 kHz, 65 kHz, and 260 kHz, respectively. Therefore, the switching frequency during light loading to provide stabilization at the predetermined voltage is 65 kHz, which is not significantly different from the switching frequency of 60 kHz during heavy loading.
Of course, if the desired switching frequency is set to be in the frequency region Wb higher than the parallel resonant frequency fp, the switching frequencies during light loading and heavy loading do not differ significantly.
Since the switching frequency during light loading is not high relative to during heavy loading, the DCxe2x80x94DC converter 10 is driven with large exciting current component, and a problem occurs in that the energy conversion efficiency of the DCxe2x80x94DC converter 10 is not improved. This problem occurs in half bridge DCxe2x80x94DC converters as well as in full bridge DCxe2x80x94DC converters.
Accordingly, it is an object of the present invention to provide a bridge DCxe2x80x94DC converter having significant improvement in the energy conversion efficiency in which the switching frequency to provide a stabilized voltage particularly during light loading is much higher than that in a conventional one.
To this end, according to one aspect of the present invention, a bridge DCxe2x80x94DC converter includes a switching unit having a pair of switching devices connected in series to form a bridge, a converter transformer having a primary coil and a secondary coil which are wound with a predetermined turns ratio for transferring to the secondary coil the switching output provided for the primary coil by the switching operation of the switching unit, a series resonant circuit having a capacitor, an inductor, and the primary coil serving as an inductor which are connected in series to a node between the pair of switching devices, the series resonant circuit being resonated at a first resonant frequency, a parallel resonant circuit at least having an equivalent capacitance in the primary equivalent to the interwinding capacitance of the secondary coil, and the inductance of the primary coil, the parallel resonant circuit being resonated at a second resonant frequency higher than the first resonant frequency, a voltage supply connected to the secondary coil for supplying an output voltage to a load, and a switching control unit for varying the switching frequency of the switching unit according to variations in the voltage output from the voltage supply, whereby a stabilized output voltage is obtained from the high voltage supply.
According to another aspect of the present invention, a high voltage generating circuit includes a switching unit having a pair of switching devices connected in series to form a bridge, a converter transformer having a primary coil and a plurality of secondary coils which are wound with a predetermined turns ratio for transferring to the plurality of secondary coils the switching output provided for the primary coil by the switching operation of the switching unit, a series resonant circuit having a capacitor, an inductor, and the primary coil serving as an inductor which are connected in series to a node between the pair of switching devices, the series resonant circuit being resonated at a first resonant frequency to provide current resonance for the switching operation, a parallel resonant circuit at least having an equivalent capacitance in the primary equivalent to a combination of the interwinding capacitances of the plurality of secondary coils, and the inductance of the primary coil, the parallel resonant circuit being resonated at a second resonant frequency higher than the first resonant frequency, a high voltage supply for coupling a voltage multiplier rectifier circuit to each of the plurality of secondary coils so as to connect them in series so that a high voltage is supplied to a load, and a switching control unit for varying the switching frequency of the switching unit according to variations in the high voltage output from the high voltage supply, whereby the high voltage output is stabilized.
Preferably, the turns ratio of the primary coil to the secondary coil is chosen so that the switching frequency ranges from the series resonant frequency to the parallel resonant frequency during heavy loading, and exceeds the parallel resonant frequency during light loading.
In addition, an additional capacitor may be connected in parallel to the secondary coil, of which the capacitance is chosen, so that the switching frequency may be set.
By determining the resonance characteristic during light loading in this manner, the switching frequency during light loading may be set in a frequency region higher than the parallel resonant frequency.
As a result, the switching frequency may be significantly high during light loading to reduce the amplitude of the exciting current during light loading, and the energy conversion efficiency is thus improved.
The bridge DCxe2x80x94DC converter according to the present invention is extremely suitably implemented as a high voltage generating circuit for use in CRTs in which a load can constantly vary in a range of heavy loading to light loading depending upon content of pictures.